The public's increasing demand for bandwidth has contributed to an acceleration in the development of wavelength division multiplexing (WDM) technology. Using WDM, data can be transmitted at a high rate on each of several wavelengths of light sharing an optical fiber. Currently, systems exist in which a single fiber carries over 100 Gb/s of data using 40 or more wavelengths.
Because of the large number of wavelengths and the high data rates involved, it becomes increasingly necessary to find more flexible means for accessing individual wavelengths of light (optical channels) for purposes of reception at, or transmission from, a given node of a WDM network. Conventionally, individual optical channels may be "dropped" by inserting a filter in the main fiber path which reflects the desired wavelength towards equipment connected to the node where optoelectronic conversion and other processing may take place. Similarly, the addition of an optical channel is typically achieved by the insertion of a filter in the main fiber path, which filter transfers light arriving at the node at a desired wavelength back into the main optical path. When multiple wavelengths are to be dropped or added, multiple optical filters must be inserted in the main optical path at the location of the node.
Since the optical add and drop filters are tuned to specific wavelengths and intercept the optical flow along the main fiber path, it is crucial to provision a conventional WDM network with the correct number, location and operational wavelength of filters upon initial installation. Conventional WDM network architectures are therefore subject to eventual fiber interruptions and to the eventual addition or replacement of equipment in order to keep pace with the evolution of the network and that of the WDM technology used to transmit data through the network.
Given the high data rates involved, any disruption of the main fiber path leads to severe inconveniences for the operator and users of the network, while the operator is further burdened with the cost of adding or replacing equipment. Clearly, what is needed is a network architecture which satisfies current operational requirements while being sufficiently flexible to accommodate evolutionary changes in the network and in the wavelength plan.